Computer-linked nuclear magnetic logging tool and method for rapidly dispersing components of residual polarization associated with a prior-in-time collection cycle as well as reduce tuning errors during ring down

ABSTRACT

The present invention decreases the time needed between collection cycles of a NML tool located in a wellbore penetrating an earth formation by zeroing the effect of prior-in-time residual polarization via a surprising change in the operating parameters of the coil circuit while guarding against the effects of tuning errors in the latter, viz., using a higher Q value than normal or its equivalent for the coil circuit during the initial stages of ringing of the coil at the proton precession frequency followed by establishing a lower Q value for the latter stages. Result: the effect is as if ringing is at the higher Q&#39; value with attendent zeroing of the residual polarization, but without the consequences of slow cutoff of the decaying magnetic field introduced during ringing. Hence, the effect of error fields associated with the latter is minimized.

SCOPE OF THE INVENTION

This invention relates to nuclear magnetic logging methods (hereinaftercalled NML methods) in which NML proton precessional signals arerepetitively collected over a series of detection periods from entrainedfluids of an earth formation by means of a NML tool within a wellborepenetrating the formation (viz., detected from hydrogen nuclei over aseries of repetitions normalized to a given depth interval).

More particularly, the invention concerns a method of reducing theeffect of prior-in-time residual polarization of a set of NML cyclicoperations associated with a common depth interval in which at least oneof the polarizing periods, of the set, is not long enough to allow suchresidual polarization to decay by relaxation before the next-in-timecollection cycle is to repetitively occur.

In accordance with one aspect, the present invention eliminates both theneed for a depolarizing period between collection cycles as well asreduces the error due to detuning of the coil circuit via a surprisingchange in the operating parameters of the coil circuit, viz., using ahigher Q value than normal or its equivalent for the coil circuit duringthe initial stages of ring down of the latter (i.e., after cutoff of thepolarizing field prior to detection of the precessing protons of thefluids of the adjacent formation) followed by permitting the coilcircuit to ring at a normal Q value for the final stages. Result: errorsdue to detuning of the coil circuit are greatly reduced but the zeroingof the prior-in-time residual polarization still occurs. Concomitantly,cyclic NML logging speed is greatly improved.

In accordance with another aspect, the invention is especially useful inNML logging situations in which the next-in-time polarizing period, isnot sufficient to bring about decay of the prior-in-time residualpolarization by relaxation without insertion of a long depolarizingperiod between collection cycles. Usual placement of the depolarizingperiods: between at least two of the series of normalized collectioncycles of descending order and/or of substantial unequal duration. Forexample, when a collection cycle with a short polarizing period followsa cycle with a long period, a portion of the polarization of theprior-in-time cycle may inadvertently be manipulated by magnetic fieldsin such a way that the polarization buildup in the next-in-time cycledoes not start at zero.

BACKGROUND OF THE INVENTION

Drillers and producers dislike the use of well-scanning tools thatdisrupt drilling and/or producing operations. They know that with thedrill or producing string pulled from a wellbore and a scanning tool inplace, many problems can arise.

For example, differential pressure at the contacting surfaces of thetool with the sidewall of the wellbore can generate a positive force asa function of time. As in-hole tool time increases, so does thelikelihood of the tool becoming struck. Also, the drilling mud getsstiffer the longer the tool is within the wellbore, and accumulations onthe top of the tool also build up. Such effects are complicating factorsfor clean removal of the tool even if the latter is continuously movingwithin the confines of the wellbore during data collection. So, the lesstime a tool is within the wellbore, the better the chances of itssuccessful removal from the wellbore--on time.

In present NML tools, resident in-hole time has been dictated byrequirements of the method itself as well as by system circuits forcarrying out the method. For example, the NML data must be collectedsuch that the effect of the polarization of the prior-in-time collectingcycle is essentially zero. Hence, either (i) sufficient time must beallowed between collection cycles, or (ii) the next-in-time polarizingperiod must be sufficiently long to establish maximum polarization ofthe entrained fluids before the NML data is collected.

Heretofore, commercial NML operations have provided sufficientconditions whereby conditions (i) and (ii) have been met. In thesimplest NML mode of operation in which NML data is collected toestablish the "free fluid index" of the formation fluids, the polarizingfield is applied to the formation a sufficient time period that maximumpolarization of the nuclei is established. That time periodautomatically guarantees that polarization of previous cycles will be atequilibrium before the proton precession signals are detected.

For cyclic NML operations, different steps are needed. A series ofdifferent polarizing time and collection time periods are used inassociation with a common given depth interval of formation (occurringin either T₁ -continuous or T₁ -stationary operations). Problems canoccur when a cycle with a short polarizing period follows a cycle with along polarizing period. As a result, polarization built up during thelong polarizing period may spill over into the short period and may bemanipulated by magnetic fields of the latter in such a way that thepolarization buildup of the latter period does not start at zero. Hence,under these circumstances, heretofore a depolarizing time interval wasinserted in the cycle of NML operations to allow the residualpolarization to decay to equilibrium by relaxation. Such depolarizingtime interval is of the order of two seconds. But since the polarizingperiods of cyclic NML operations is each only a few tenths of a secondand the signal observation intervals each is likewise only a tenth of asecond or so, the need for such a long depolarizing period has imposedsevere limitations on NML logging speed, say to about 300 feet/hour. Ithas only been tolerated because of the large time requirements of theuphole computer linked to the NML tool, viz., the time needed by thatequipment to reduce the NML data to an acceptable display form. I.e.,such reduction (being of the order of two seconds to reduce the observeddata to an acceptable display form) has allowed time for the priorresidual polarization to decay to equilibrium before the shorterpolarization cycle was implemented.

However, now improvements in hardware and software within the associateduphole system at the earth's surface have been proposed by oil fieldservice companies. Goal: to reduce the time frame needed for thecomputer to reduce the NML data to acceptable form between collectioncycles. Such advances encompass hardware, software and/or firmwareimprovements from individual as well as various combinational forms.However, I have found that advances, the total time required forperforming a set of collection cycles of different polarizing periods(even though combined in a collection process that uses theabove-mentioned proposed improvements), remains about the same aspreviously practiced. Reason: in cyclic NML logging, a 2-seconddepolarizing period must be used between selected collection cycles toinsure that the polarization buildup always begins at zero. The speed ofthe logging sonde under these circumstances: about 300 feet/hour.

These limitations also apply regardless of how long the polarizing timesare, or how the ratios of the polarizing periods relate one to theother. For example, in reference to the former in practicing T₁continuous logging even if the polarizing periods of a normalized set,were changed to 3200, 800, and 1600 milliseconds, the total time persequence would still take 6 seconds, even if the polarizing periods werechanged to 3200, 800, and 1600 milliseconds, the total time per sequencewould still take 6 seconds even though there is no need to insertdepolarization periods between cycles. Or, in reference to the latter,instead of polarizing times of the set defining ratios of 4:1:2,different ratios, say 20:1:4 (viz., a set of polarizing times of 2000,100, and 400 milliseconds with each followed a short signal-observingperiod) requires about 5 seconds per sequence, since a 2-second time fordepolarization must be used after the 2000-millisecond polarizingperiod. Result: little improvement in logging speed.

Hence, there is now a need to artificially dispose of the effects ofprior-in-time polarization within a period substantially shorter thanthe typical 2-second maximum mentioned above under normal NML cyclicoperations and preferably within a period shorter than the presentsignal-observation time (viz., shorter than above one-tenth of asecond). In that way, there would be provided a significant improvementin NML logging speed, e.g., say from 300 to about 500 feet/hour.

Hence, an object of this invention is to provide a method of reducingthe effect of residual polarization in cyclic NML operations normalizedto a common depth interval whereby such polarization can be reduced toapproximately zero within a time period less than the present signalobservation time, viz., less than approximately 100 milliseconds.Result: the repetition rate for a series of NML collection cyclesnormalized to the same depth interval is much improved and loggingoperations can be carried out at a surprisingly rapid rate.

SUMMARY OF THE INVENTION

In accordance with the present invention, I have discovered that thesignal contributions from residual polarization can be manipulated--toreduce the required depolarization period to the above desired rangecentered at about 0.1 second--without extensive modification of existingcircuitry of the NML tool. In accordance with the method, the NMLpolarizing and detection circuitry of the tool is tuned to approximatelythe resonant frequency of the expected NML precession signals as well astuned to maximize enhancements brought about as the coil circuit ringsat a selected Q value or quality factor after cutoff. In this aspect,the term "tuning" is used both to describe the altering of the values ofcircuit elements of the coil circuitry to achieve resonance at a desiredfrequency in the frequency sense as well as to describe similar changesto achieve enhancement of the polarization as the ringing fieldundergoes damped oscillation as a function of time in the Q-sense.

The present invention thus has special application in NML operations inwhich after cutoff of the polarizing field the coil circuit is permittedto ring at the proton precession frequency. That is to say, as thepolarizing current is cutoff, the collapse of the polarization fieldcauses an oscillating voltage to be generated in the coil. A large partof this voltage and the resulting current (representing a quantity ofenergy stored in the polarizing circuitry) is dissipated within the coilcircuit. A small part, however, (called "ringing") is permitted todecrease with time and produce an oscillating resonant magnetic field atthe proton precession frequency that propagates outward into theformation and reorients the polarization previously produced. The rateof decay of the ringing is a function of the Q of the circuitry. For anycombination of system parameters, including coil configuration, boreholediameter and position of the coil in the borehole, thus a normal Q ofthe circuitry exists that produces a maximum NML response after thepolarizing field collapses and ringing occurs.

In other words, for a given set of system parameters, there is a Q inwhich reorientation of the produced polarization is effected--withenhancing advantage by the magnetic field generated during ringing.

While heretofore the relationship of the collapse of the polarizingfield upon the polarizing coil, resulting ringing of the polarizingcircuitry and the tuning of the Q of that circuitry for enhancementpurposes has been established, I have now discovered that instead ofchoosing the Q of the circuitry based on maximizing the resulting NMLsignal response, that a better criterion is to minimize the effect ofprior-in-time components of residual polarization that were parallel tothe earth's field at the start of the polarizing period of interval.This has been found to be surprisingly easily to bring about by merelyshifting the Q of the polarizing circuitry or its equivalent parametersto a higher artificial Q level than heretofore practiced.

Such shifts in Q value or its equivalent parameter is based in part (i)on the discovery that the signal strength of the desired polarization asa function of Q is asymmetric wherein the slope of the trailing edge athigher Q's over a predetermined segxent, measured from the maximumresponse to say 2% down therefrom, is surprisingly shallow, and (ii) onthe discovery that signal strength of the prior-in-time residualpolarization undergoes a phase reversal at a higher Q' value within theaforementioned predetermined segment. But I have also discovered thatwhen the coil circuit is permitted to ring at such a higher Q' value,complete zeroing of the residual polarization may still not occur due totuning errors introduced by the coil circuit and the full NML signal maynot be later detectable. However, such tuning errors can now bemitigated by establishing a low Q value for the coil circuit after theinitial stages of ringing has occurred at the artificially higher Q'value. Result: the effect is as if ringing is at the higher Q' valuewith attendent zeroing of the residual polarization, but without theconsequences of slow cutoff of the decaying magnetic field introducedduring ringing. Note that such damping preferably occurs after one- ortwo-time constants of ringing and can also be useful in minimizingeffects due also to frequency-tuning errors during normal signalgathering cycles.

The mechanism of disposal of the prior-in-time residual polarization isas follows:

If the signal from the prior-in-time polarization period is plotted as afunction of Q during ringing, and the Q for the operating system is thenchosen about the zero crossing of the prior-in-time signal, there is asubstantial reduction in signal strength of such polarization due to thecancellation of parallel components. That is to say, by the deliberatemistuning of the polarizing circuitry to a higher Q' value, cancellationof the parallel components of the prior-in-time polarization occursabout the zero crossing point.

But such cancellation occurs in accordance with present inventionwithout the introduction of an error field due to detuning errors in thecoil circuit.

In the present invention, the increases in the Q of the polarizingcircuitry are in range of 18 to 35% of the normal Q value for maximumNML precessional response. An increase of about 25% is preferred.

Preferred time frame for establishing the low value for the coil circuitafter the initial ringing at the higher Q' value: about 1/2- to two-timeconstants of the field generated during ring down.

In accordance with the present invention, the omission of a previouslyrequired depolarizing period results not through dispersal of theparallel components of residual polarization, but rather via theirmanipulation so that roughly as much signal is produced in phase withthe legitimate signal from the next-in-time polarization as is producedin the opposite phase.

But in addition, the introduction of error magnetic field due to thedetuning of the coil circuit as the latter rings at the higher Q' valuehas no (or at least very little) influence on the precessing protons ofinterest in the formation since to them, the cutoff of the ringing fieldappears to be sudden.

However, after the ringing has decayed, the Q of the system as seen bythe detection circuitry is normally increased for the reception of theNML signal. Such increases still occur in accordance with the presentinvention. Also, the commercial NML operates with a single coil systemfor both the polarizing and signal reception operations, but theextension of these descriptions of the invention to a system withseparate coil systems for the two functions is quite clear.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partial schematic of an improved NML system in awell-surveying environment wherein an uphole computer-linked control andsignal-generating and recording circuitry is depicted in system contactwith nuclear magnetic polarizing and signal detection circuitrypositioned within a borehole penetrating an earth formation;

FIG. 2 is a diagram illustrating the polarizing and signal detectioncircuitry of FIG. 1 in which a damping resistor is disconnectablyconnected in parallel with a tuning capacitor and with a singlepolarizing and detection coil;

FIG. 3 is a vector plot of the earth's field (B_(e)), the polarizingfield (B_(p)) and the resultant field (B=B_(e) +B_(p)) at the start ofcutoff of the polarizing current and ringing of the coil of FIG. 2,illustrating the theoretical basis of the present invention;

FIG. 4 is a plot of the angle φ_(e) between the earth's field (B_(e))and the polarizing field (B_(p)) of FIG. 3 and the angle θ between thepolarization M and the resultant field (B) as a function of differentdimensionless parameter values A;

FIG. 5 is the angle φ as a function of angle wherein phase angle (ψ),angle θ between the polarization M and the resultant field (B) and sin θare plotted to show their interdependence;

FIG. 6 is a plot of cutoff efficiency as a function of dimensionlessparameter A for a series of different coil circuits again useful inexplaining the theoretical basis of the present invention;

FIG. 7 is a schematic circuit diagram that focuses in more detail on theoperation of the coil circuit of FIG. 2;

FIG. 8 is a circuit diagram akin to FIG. 7 illustrating an alternativeto the circuitry of FIG. 2;

FIG. 9 is another plot of cutoff efficiency as a function ofdimensionless parameter A for a coil circuit having different Q values;

FIG. 10 is a graph of precessional signal strength as a function of Qvalues of three different signals associated with a series ofpolarization repetitions normalized to a common position relative to theborehole of FIG. 1, in which relaxation is ignored but stillillustrating that residual polarization from the prior-in-timepolarization period can be minimized without adverse reduction innext-in-time precessional signal strength;

FIG. 11 is a series of wave form diagrams of the three prior-in-timesignals of FIG. 10 illustrating how residual polarization varies as afunction of Q and thus can be minimized by use of a detuned polarizingcircuit whose Q is matched to that which occurs at the zero crossing ofthat signal;

FIG. 12 is a plot of angle θ between the polarization M and theresultant field (B) as a function of the dimensionless parameter A'illustrating the effects of mistuning;

FIG. 13 is a modification of the coil circuit of FIG. 2 for mitigatingagainst the effect of the tuning errors;

FIG. 14 is another plot of θ as a function of dimensionless parameter Aillustrating how the circuit of FIG. 13 reduces the effect of tuningerrors.

DESCRIPTION OF THE INVENTION

As shown in FIG. 1, a logging sonde 10, shown in phantom line, ispositioned within a borehole 11 penetrating an earth formation 12.Within the sonde 10 is polarizing and detection coil circuitry 13.Purpose of circuitry 13: to polarize the adjacent formation 12 and thendetect NML precessional signals from hydrogen nuclei of entrained fluidson a cyclic basis normalized a series of depth interval along theborehole 11. A typical depth interval is shown at 14 wherein a series ofpolarizing periods, followed by shorter detection periods, provide asequence of NML data associated with a conventional series of collectioncycles. Typically, in a T₁ collection sequence a set of numbered,different collection cycles (say 100, 200, 400, 800, 1600 and 3200millisecond periods) are internally repeated a number of times (usuallyabout ten times) all normalized to the same depth interval whereinsimilar resulting data can be stacked. But higher precision of theinternally-stacked data requires more complete disposal of the residualpolarization to prevent an incremental gradual buildup of the latter,even though the internal polarizing times are equal. Hence, one or morepolarizing periods had to be allowed for within each collection cycle.Thereafter, the resulting NML data is sent uphole via logging cablegenerally indicated at 15 that includes a strength-aiding elements (notshown) rigidly attached to the sonde 10 (at the uphole end thereof) androtatably supported at the earth's surface 16 by a support winch 17 thatincludes a depth sensor (not shown). Cable 15 also includes a series ofelectrical conductors generally indicated at 18 for aiding in thecontrol and operation of the downhole polarizing and detection circuit13. For example the series of electrical conductors 18 can include a NMLsignal transmitting conductor 18a by which the NML data collecteddownhole can be transmitted uphole for enhancement and recording withina computer-controlled recording system also at the earth's surface 16and generally indicated at 19. Such computer-controlled recording system19 includes a computer within computer-controller 20 whereby theresulting NML data can be organized and enhanced to provide meaningfulestimates of permeability and porosity of the given depth interval.Results are recorded at recorder 21 as a function of depth intervalalong the borehole 11 as determined by the depth indicator at winch 17.Also linked to computer-controller 20 is a signal-forming and drivingnetwork 22 comprising pulse generator 23, gate 24 anddigital-to-analogue convertor 25. As explained in more detail below, thenetwork 22 provides a tailored polarizing current each collection cyclethat is transmitted downhole to the polarizing and detection circuit 13within sonde 10, say via a pair of current conductors 18b and 18c shownattached to relay 26 at the output of D/A convertor 25 of signal-formingand driving network 22. Also located at the earth's surface 16 are aseries of additional electrical conductors 18d, 18e, 18f, and 18g. Theseadditional conductors connect to computer-controller 20 of thecomputer-controlled recording system 19 for the purpose of controllablylinking various elements, miscellaneous equipment and operations of thepresent invention at both the earth's surface 16 and downhole withinpolarizing and detection circuit 13 as explained in more detail below.For example, conductor 18d aids in the control of uphole relay 26 duringoperations so that after polarization, the signal-forming and drivingnetwork 22 is disconnected from the downhole polarizing and detectioncircuitry 13 for more reliable detection of the precessional NML signalduring each collection cycle.

Computer-controller 20 can include improvements in hardware and softwarewithin the associated computing system as proposed by oil servicecompanies. Goal: to reduce the time needed to reduce the NML data to anacceptable display form between collection cycles. However, even usingsuch advances, the total time required for performing say T₁ stationarymeasurements has remained the same as before practiced due to residualpolarization buildup. The speed of the logging sonde 10 under thesecircumstances: about 300 feet/hour.

FIG. 2 illustrates the polarizing and detection coil circuitry 13 inmore detail.

As shown, a polarizing coil 35 is connected uphole to signal-forming anddriving network 22 as follows: on one side by conductor segment 36a,switch 37, and uphole conductor 18c, and on the other side by conductor18b so as to be driven with a polarizing current of predeterminedduration. In series with conductor segment 36a are conductor segments36b, 36c, 36d . . . 36f. These latter conductor segments 36b . . . 36fconnect the coil 35 to the active section of signal detection circuit 38during the detection of the precessional NML signals. The coil 35 isalso connected to ground at 39 within detection circuit 38 by means ofadditional conductor segments 40a, 40b . . . 40d. Between the conductorsegments 36b . . . 36f and ground 39 are several circuit elementsparalleling the coil 35. Paralleling the coil 35 are Zenes diodes 42, aresistance element 43 and a capacitor 44. Switch 45 connects resistanceelement 43 to ground 39 while switch 46 disconnectably connectsconductor segments 36e and 36f. Switches 45 and 46 are controlled bycommand signals originating uphole and passing thereto via conductors18f and 18g, respectively, during polarization, cutoff of the polarizingcurrent and ringing of the coil 35 as well as during detection of theprecessional NML signals. In addition, during cutoff and ringing, theresistance element 43 and the coil 35 are used to establish damping ofthe collapsing field as explained in more detail below.

During polarization, uphole relay 26 of FIG. 1 and switches 37, 45, and46 of FIG. 2 are controlled so that the coil 35 is driven by polarizingcurrent from the uphole circuitry at a maximum level; at the same time,the detection circuitry 38 of the polarizing and detection coilcircuitry 13 is protected. Referring to the Figures in more detail, thecontacts of relay 26 and switch 37 are initially controlled so as to beclosed during polarization. In that way, the uphole signal-forming anddriving network 22 is contacted directly to the coil 35. At the sametime, the contacts of switch 45 and of switch 46 remain open during thisperiod of operations, i.e., remain open with respect to upholeconductors 18b and 18c. As a result, the coil 35 is driven with amaximum polarizing current to generate a strong polarization fieldoriented at an angle to the earth's field over a predetermined timeduration without harming the detection circuit 38. Thereafter, thecontacts of the uphole relay 26 and switch 37 are opened as switch 45 isclosed and the polarization field of the coil 35 is permitted tocollapse. During collapse of the field, a discharge path is initiallyestablished through the Zener diodes 42 to ground 39 for theself-induced current within the coil 35 wherein the self-induced voltageacross the diodes 42 remains essentially constant for any value ofinduced current above a selected value for a selected time frame. Ofcourse, the most rapid field decay would be obtained with the coil 35unloaded, i.e., with a substantially infinite resistance path beingplaced in parallel with the coil 35. Unfortunately, the magnitude of thevoltages induced would destroy not only the coil 35, but also theassociated electronics. On the other hand, a low resistance across thecoil 35 that would maintain the transient voltage within reasonablelimits would make current cutoff too slow for detection of precessionalsignals.

But in accordance with the present invention because an initialdischarge path for self-induced current, as provided by the Zener diodes42, is followed by generating an enhancing oscillating field by drivingthe coil with a current inversely proportional to resistor 43 connectedin a parallel circuit with the coil 35 but dissipating. Result: enhancedringing of the coil at the proton precession frequency within a timeframe that still allows for timely detection of the precessional signalsfrom fluids within the formation at modest cutoff rates. In this regard,the resistance value of resistor 43 is selected at a value thatassociates the Q value of the coil circuit as explained in more detailbelow wherein ringing of the coil (at a higher Q value) is carried outat the frequency of proton precession of the adjacent fluids.

It should be noted in this regard that during damping of the storedenergy, that although higher resistance value of the Zener diodes 42results during fast cutoff, this effect is overshadowed by the value ofthe resistance element 43 in parallel with tuning capacitor 44 inestablishing the higher Q of the coil 35. Such a Q value permits thecoil circuit to quickly approach a condition that allows magneticoscillations or "ringing" to occur as the stored energy is damped forenhancement of the previously generated nuclear polarization, asexplained below.

After ringing has subsided, switch 46 is closed as switch 45 is opened.This operation connects the coil 35 to the initial stage of the signaland detection circuit 38. During this time frame, the capacitor 44 (withthe resistor 43 decoupled) continues to tune the coil 35 to the protonprecession frequency of the NML signals to be detected. With theresistor 43 decoupled from the circuit, the Q value of the circuitry islowered. This occurs even though the Zener diodes 42 are still in thecircuit but the latter have no effect, since they appear as an almostinfinite resistance to the low voltages of the circuitry. Note, also,that the Q of the coil circuitry can even be changed, if desired, byappropriate operation of the feedback networks associated with theamplifying network of the circuit 38. The theory and description of suchnetworks are discussed in detail in my U.S. Pat. No. 3,204,178 for"AMPLIFIER INPUT CONTROL CIRCUITS", issued Aug. 31, 1965. Such networksprovide for reasonably low resistance paths for discharge of theself-induced voltages while the Q of the circuitry is changed to a valuecompatible with effective reception of the precessional signals.

While general equations of state for polarization during cutoff areavailable at least for the case of a polarizing field that starts largecompared to the earth's field and is then bought linearly to zero (viz.,instantaneous cutoff), none have been developed for the case in which a"ringing" oscillating field is used to enhance the previously generatednuclear polarization at the proton precession frequency of fluids to bedetected in an adjacent earth formation. Briefly, I have found thatallowing the coil 35 to ring at the frequency of proton precession withan appropriate higher Q following cutoff provides for signal detectionsensitivity that is only slightly less than that obtained withinstantaneous cutoff, but also minimizes the effects of prior-in-timecomponents of residual polarization that were parallel to the earth'sfield at the start of the polarization period. The basis for thisconclusion is set forth below in which a series of key NML terms aredeveloped in conjunction with the definitions set forth below in theSection entitled "SYMBOLS AND DEFINITIONS SECTION", viz.:

Nuclear Magnetization, Dipole Moments Polarization and Relaxation

Hydrogen nuclei of entrained fluids of the earth formation have magneticdipole moments which produce magnetic fields somewhat like those of tinymagnets. Were it not for the fact that the moments can come within theinfluence of the polarizing field of coil 35, their fields would berandomly oriented and not produce an observable external magnetic field.But since they are subjected to such field, their associated magneticfields can become aligned with that field. At the same time, ascrambling effect due to thermal motion is produced. It tends to preventsuch alignment. But a slightly preferential alignment (calledpolarization) occurs. Note that the polarization is proportional to thestrength of the polarizing field that causes the alignment but inverselyproportional to absolute temperature, the latter being a measure ofthermal motion tending to scramble the system of nuclear magneticmoments.

The nuclear polarization produces a magnetic field which can bedetected. Note that the polarization does not decay immediately when thefield is removed. The process of the approach of the polarization to itsnew equilibrium value when the magnetic field is changed is called"relaxation" and the corresponding times are called "relaxation times".(Note in this application the term "nuclear magnetization" correspondsexactly to polarization but it is acknowledged that the latter issometimes referenced as a dimensionless term only.)

Precession

In addition to being little magnets, fluid nuclei are also like littlegyroscopes, and can be twisted just as gravity twists a spinning top.Result: The nuclei precess. That is, they precess unless they arealigned with a strong field just as the toy top precesses so long as itis not aligned with the earth's field of gravity.

Detection of Precession

A precessing nuclear polarization produces a rotating magnetic fieldwhich in turn generates electric signals which can be detected.Precessional frequencies are directly proportional to the strength ofthe twist causing the precession, that is to say, it is directlyproportional to the strength of applied field, and the precessionalfrequency is 4.2577 kilohertz per gauss of applied DC field for hydrogennuclei of interest.

Conditions for Precession

Two things must be present to obtain a precessing polarization. First,the polarization must be produced by subjecting the fluids to apolarizing field for an approprite length of time. Second, thepolarization and another field must somehow be made not parallel to eachother as by reorienting the fields so that the polarization is subjectedto a magnetic field in a new direction.

In nuclear magnetism logging, proton precession is caused to take placein the earth's field after the nuclear polarization has been generatedin a direction in the borehole at an angle, say preferably 90° to theearth's field when the polarizing field is cut off, the polarization isleft to precess about the earth's field.

CUTOFF EFFICIENCY

Consider the case of an idealized two-dimensional dipole, single-coillogging system centered in a borehole. The units used here are intendedto be consistent and, for convenience, several quantities will be usedin dimensionless form, and certain other conventions will be adopted, asset forth under the "SYMBOL AND DEFINITIONS SECTION" infra. Withinstantaneous polarizing field cutoff, the signal from a small elementof area is proportional to the local value of sin² φ_(e), the anglebetween the earth's field (B_(e)) and the polarizing field (B_(p)) andinversely proportional to the fourth power of l, the distance from theborehole axis. As shown in FIG. 3, B_(p) is the polarizing field,indicated by vector 50, and B_(e) is the earth's field, indicated by 51.A bar or caret over the symbol for a field indicates a vector, (with thecaret indicating a unit vector) absence of a bar or caret indicates thescalar amplitude, and a dot over a symbol indicates rate of change.Thus, -B_(p) is the rate of reduction of the polarizing field. The rateof increase of φ, the angle between the polarizing field B_(p), andresultant of B_(p) and the earth's field, B_(e) indicated by vector 52,is governed by the ratio B_(p) /B_(e). If B_(p) is constant duringcutoff prior to ringing, if any, the cutoff can be characterized by thedimensionless parameter

    A'=(-B.sub.p /B.sub.e⊥)/ω.sub.e⊥ =B.sub.p /(γB.sub.e⊥.sup.2),                            (1)

where ω_(e)⊥ is the instantaneous precession angular frequency whenφ=90°, and γ is the magnetogyric ratio. The ratio -B_(p) /(γB_(e) ²)=G',with G'=A' sin² φ, is fixed if the only parameter varied is φ. Much ofour discusslon will be specialized to the case φ=90°, for which A'=G',i.e., the NML tool is entered in the borehole with axis, of the latterparallel to the earth's field.

In NML, B_(e) is usually of the order of a half gauss, and B_(p) at theedge of the borehole, usually over a hundred gauss. The time to turn offB_(p) is of the order of 10 milliseconds. Thus, G at the edge of theborehole may be of the order of two, and it decreases rapidly withdistance into the formation.

The instantaneous rate of cutoff will be characterized by the parameter

    α=Ω/ω=A' sin.sup.3 φ=G' sin.sup.3 φ/sin.sup.2 φ.sub.e,                                              (2)

where Ω=dφ/dt, and ω is the instantaneous angular frequency ofprecession about the resultant of the earth's field and the polarizingfield. If α>>1, cutoff is fast, and the polarization is nearly leftbehind as the direction of the resultant field changes. If α<<1, thepolarization nearly follows. Reference to FIG. 3 shows that for most ofthe cutoff time, the angle φ is very small. For instance, if A'=2, andφ=20°, Equation 2 gives α=0.08. Even for a much higher cutoff rate, thefirst ten or more degrees of angle change is slow. The rest of the cycleis in the intermediate range, with α comparable to one, unless φ_(e) isnearly 180°, in which case the precessing polarization is not coupled tothe NML coil to give a signal.

To specify the position of polarization during cutoff, θ is defined asthe angle between the polarization M and the resultant field B=B_(p)+B_(e), and the phase ψ is specified as the angle about B with respectto the plane of B_(p) and B_(e), viz., in the plane of FIG. 3.

With non-instantaneous cutoff, the signal contribution from an elementof area is proportional to sin φ_(e) sin θρ⁻⁴ e^(i)ψ, where θ is theangle between the polarization and the earth's field after cutoff. Thecoupling to the coil remains proportional to sin φ_(e), and this factoris not affected by cutoff rate. Since a factor of sin θ replaces afactor of sin θ_(e), when cutoff is not instantaneous, the factor of sinθ/sin φ_(e) is regarded as relevant to cutoff efficiency. Note that thisfactor can exceed 1.0. The observed signal is the sum of contributionsfrom all elements of area from the borehole wall to infinity, added withproper regard to the phase, ψ. The cutoff efficiency is then theabsolute value of the signal with the actual mode of polarizing fieldcutoff divided by the signal with instantaneous cutoff: ##EQU1## where ηis azimuthal angle around the borehole axis. (Note that the customarysymbol, φ, has already been used for something else.) Note also that θand ψ depend φ_(e) and ρ. For the geometry under consideration, φ_(e)does not depend on ρ or A, but may depend on η. The integrations in thedenominator are separable.

The local cutoff rate A is a function of the strength of the polarizingfield, which is inversely proportional to the square of ρ: ##EQU2##

In the following, the symbol A' is used to indicate A(ρ) at a generaldistance into the formation and the symbol A or A(o) is used to indicateA(a), the value of A at the borehole wall.

Substituting (5) into (3) ##EQU3##

Let the ratio of the A-integrals be E*(η), so that E*(η) is given by

    E*(η)=<sin φ.sup.iψ >/sin φ.sub.e          (7)

where the brackets <> indicate an average with respect to A' over therange from zero to A.

Put (7) into (6)

    E=|<sin.sup.2 φ.sub.e E*>|/<sin.sup.2 φ.sub.2 >(8)

where here the <> indicate average with respect to η over the intervalfrom zero to two π.

If the earth's field is parallel to the borehole axis, then sin φ_(e)and E* are no longer functions of η. Then the cutoff efficiency issimply

    E=|E*|=|<sin  θ e.sup.iψ >|(9)

Recall that the position of the polarization can be specified by θ (θ isthe angle between the polarization M and the resultant field B=B_(p)+B_(e)) and ψ where ψ is the angle about B with respect to the plane ofB_(p) and B_(e), viz., in the plane of FIG. 3. If α and its firstseveral φ-derivatives are much less than one, the approximatepolarization positions are given by

    θ=tan.sup.-1 α=tan.sup.-1 A'

    ψ=π/2-tan.sup.-1 (dα/dφ)                  (10)

As before, the primes indicate a general position in the formationrather than values at the edge of the borehole. From (9), ##EQU4##

From (10), the obtained values are used as starting points for numericalintegrations to compute θ and ψ for the part of the cutoff for which αis not very small and are plotted in FIG. 4.

FIG. 4 is a series of curves 55 showing the buildup of the angle θduring cutoff as a function of angle φ_(e) Curve maxima are connectedalong dotted line 56. Note that the curves 55 are for constant A'=G'=α;i.e., constant B_(e)⊥. Also note that curve segments to the right ofdotted line 56 are not usable for signal because of rapid phase changes.

FIG. 5 is a series of curves 57 to show the interdependence of φ, ψ, andsin θ as a function of φ for constant A'=0.75, thus showing the effectof various angles between the earth's field and polarizing field on theformer. Precessing polarization is proportional to sin θ, and itscoupling to the NML coil is proportional to sin φ_(e). In NML, if theborehole axis is parallel to the earth's field, much of the signal comesfrom regions where φ_(e) is close to 90° (π/2 radians). If the anglebetween the axis and the earth's field is 30°, most of the signal comesfrom regions with φ_(e) between 60° and 120° (π/3 and 2π/3 radians).From FIG. 5, it is seen that this range of angles does not drasticallyreduce the signal below that which would be obtained with φ_(e) veryclose to 90°. I.e., sin θ curve 57a is roughly linear in the vicinity of90°, giving an average only a little lower than for 90° after putting inthe coupling factor. The phase differences are mild. If the earth'sfield is perpendicular to the borehole axis, about half the signal islost.

Similarly, from Equation (9), values of E can be obtained by computerwithout making the approximation (11). Such values are shown in FIG. 6as a series of curves 58a, 58b, and 58c. Curve 58a shows E values for acoil circuit like that of FIG. 2, except it has been critically dampedduring cutoff; curve 58b is for a coil circuit in which the resistor 43of FIG. 2 has been placed in series with the coil 35; and curve 58cillustrates E values for a coil circuit that has been designed toprovide linear cutoff and no overshoot.

Equation (11) may be altered by somewhat compressing A for larger valuesto give a good fit to the computed values for larger A. ##EQU5## At A=0,

    (dE/dA)=1/2; (d.sup.2 E/dA.sup.2)=0                        (12a)

ENHANCEMENT OF θ BY RESONANT PULSES AFTER CUTOFF WITH VERY SMALL A'WITHOUT MINIMIZING RESIDUAL POLARIZATION

For very small A', M is nearly parallel to B_(e). If an oscillatingfield is applied parallel to B_(p), this field can be resolved intocomponents parallel B_(e) and parallel to B₂ (i.e., perpendicular toB_(e)). Note in this regard that B, and B₂ are defined as follows: B₁=B_(e) ×B_(p) and B₂ =B₁ ×B_(e). Further, the component parallel can beresolved to B₂ into two components rotating in opposite directions inthe plane of B₁ and B₂, each with amplitude half that of the componentparallel to B₂. Then

    B'.sub.rot =1/2B'.sub.osc sin φ.sub.e                  (13)

If B'_(rot) is substantially smaller than the earth's field and if thefrequency of the field is the proton precession (Larmor frequency), thecomponent of the field parallel to the earth's field and the rotatingcomponent, whose sense is opposite to that of the proton precession, canbe ignored.

If the polarization is viewed from a frame of reference rotating aboutB_(e) with the earth's-field precession frequency, it is seen that asecondary precession at rate B'_(osc) about some axis in the plane of B₁and B₂ (i.e., perpendicular to B_(e)). The position of this axis dependson the phase of the oscillating pulse with respect to the time one jumpsonto the rotating reference frame. If B'_(osc) is of fixed amplitude andis applied for a time Δt, the polarization is rotated by an angle

    μ=B'.sub.rot Δt                                   (14)

If θ is small at the end of cutoff with very small A, we have at the endof the oscillating pulse θ≃μ'.

The concept of a rotating frame of reference has been discussed in myprior patents with respect to NMR response of drilling chips. Adifference in the NML application is that the oscillating pulse is notnecessarily perpendicular to the precession field and that in the NMLthe strength of B'_(osc) varies within the sample from zero to somemaximum value instead of being substantially constant over the sample.

Since μ' is proportional to G', the average indicated in (9) may betaken with respect to μ' instead of A'. Note that in our approximation,ψ is constant and can be ignored for the purpose of calculating cutoffefficiency. Thus, we have ##EQU6##

This function has a maximum of E_(m) at μ=μ_(m), where

    E.sub.m =0.7246114

    μm=2.331122 radians=133.5635°                    (16)

Note that the value of the current in the coil necessary to produce arotation μ at the edge of the borehole is a function of the boreholesize in the case of a centered coil system. That is, one would need to"tune" the oscillating current to the borehole size.

The computation of this section shows that a cutoff efficiency of 721/2%is attainable even with very slow cutoff. However, there are severaldisadvantages to slow cutoff because of relaxation, which is ignored inthe definition of cutoff efficiency. We will see in later sections thatone can use different (usually lesser) values of μ to enhance cutoffefficiency even when G is not small, with cutoff efficiencies somewhathigher than E_(m).

RING DOWN WITHOUT MINIMIZING RESIDUAL POLARIZATION

The oscillating pulses discussed in the last section are provided bycausing the coil 35 to ring. If separate polarizing and receiving coilsare employed, either or both coils could be used, either simultaneouslyor in sequence.

In order to provide the oscillating pulses, it is preferred to cause thecoil 35 to ring when tuned to the proton precession frequency. IfB'_(rot) is not constant in time, the generalization of (14), ##EQU7##The ringing method of applying oscillating pulses has the advantages ofconvenience, of not having to disconnect the tuning condenser, and of apulse form not ending with a switching disturbance.

RING DOWN WITHOUT MINIMIZING RESIDUAL POLARIZATION VIA SIMPLE PARALLELCIRCUIT

FIG. 7 shows a simplified basic NML single-resistor 60 in parallel withcoil 61 and its turning condenser 62, like that of FIG. 2, in which thepolarizing field has been cut off via opening the contacts of switch 63.The value of condenser 62 tunes to the nuclear precession frequency whenthe damping resistor 60 has been disconnected. Also in the circuit is avoltage limiter 64, which limits the back-voltage during polarizingcurrent cutoff to some definite value. This limiter takes the form of apair of Zener diodes. The resistance value R of resistor 60, whichlowers the Q of the input circuit during cutoff, would presumably bedisconnected after cutoff and before signal observation. This isachieved by deactivating switch 65 during such detection period. If widebandwidth is desired during signal observation, one would presumablyaccomplish this negative feedback rather than the introduction of anadditional source of noise in the circuit.

The polarizing current is assumed to be held constant up to some chosentime, at which the source of current is removed (shown symbolically byopening the contacts of switch 63), and the current through coil 61 flowfor a time through voltage limiter 64 and the resistor 60. While currentis flowing through voltage limiter 64, the voltage is constant acrossthe coil 61, and the current through the resistor 60 is also constant.The current through the coil 61 decreases linearly (the rate being theratio of the voltage across the limiter 64 to the coil inductance) untilthe current through the limiter 64 reaches zero. This instant is definedas time-zero, or t=0. The voltage limiter 64 is assumed effectively outof the picture after this time. The current through the coil 61 beforethis time is ##EQU8##

By noting the definition of G and A in the definitions section, thepolarizing field at the edge of the borehole is given by

    B.sub.p ≅-8 B.sub.e.sup.2 Ad≅-At t≦0 (20)

since Q=R/X for the parallel circuit, with X=ω_(o) L, and ω_(o) havingunit in the system of units given in the above section. Again, note thatto refer to general positions in the formation instead of the edge ofthe borehole merely need add the "prime" symbols to B_(p), G, A, etc.

Since ω_(o) =(LC)^(-1/2) and X=(L/C)^(1/2), the resonant angularfrequency for noninfinite Q is ##EQU9## After time-zero, the inputcircuit will ring with time constant 2Q (i.e., 2Q/ω_(o)). The transientamplitude and phase are determined by matching the amplitude and slopeof Equation (20) at t=0. The solution is, for t≧0, ##EQU10##

In the special case of critical damping Q=1/2 for t≧0 ##EQU11##

RING DOWN WITHOUT MINIMIZING RESIDUAL POLARIZATION VIA SIMPLE SERIESCIRCUIT

FIG. 8 shows a simple series circuit in which the Q after cutoff, viaopening the contacts of switches 59a and 59b, is determined by theresistor 66 in series with tuning condenser 67. In this case, thecurrent through coil 68 is zero at the time when voltage limiter 69drops out of the picture. After the transient has decayed and beforesignal observation, presumably the resistor 66 is shorted by thecondenser 67. Here, the phase of the transient is simple. ##EQU12## Theangular frequency ω is still given by (21).

In the case of critical damping (Q=1/2), ##EQU13##

SLOW CUTOFF WITH θ ENHANCEMENT BUT WITHOUT MINIMIZING RESIDUALPOLARIZATION

If Q is of the order of 2.0 or more, the difference between (22) and(26) is mainly a phase shift by an angle of the order of 1/Q, or, in ourunits, a shift of time-zero by about 1/Q. Thus, for Q≧2, similar resultsfor the parallel and series arrangements, is expected.

For smaller Q, the situation is very different. In the case of criticaldamping by the parallel resistor 60 of FIG. 7, the current through thecoil 61 never reverses, and the cutoff efficiency is much less than withsimple linear cutoff alone. Here, for very small A, the conditions forvalidity of (31) are fulfilled for the entire current decay, that is, tothe point where α is zero. Thus, one expects the signal to be of atleast second order in A for small A. Furthermore, in the criticallydamped parallel circuit, the rate of cutoff during the important timewhen the field is reduced from about the strength of the earth's fieldto zero is limited by the parallel circuit in such a way that increaseof A has almost no effect for A greater than about one.

On the other hand, in the series-damped circuit of FIG. 8, the coilcurrent is affected by neither the condenser 61 nor the resistor 60until the coil current has been reduced linearly to zero. Then, even forcritical damping, there is a current undershoot which enhances the angleθ for small or moderate A.

For small A in the case of the series circuit (or either circuit if Q isof the order of two or greater), the polarization at the end of thelinear portion of the cutoff is nearly in the B₁ direction,

    M·B.sub.1 ≃A'                       (28)

Consider the simple series circuit of FIG. 8 and at t=0 and adopt therotating frame of reference mentioned previously. Then the effectivefield is in the B₁ direction. For (13) (17), and (18,

B'_(rot) =(1/2)A' e^(-t/)(2Q) (29)

    μ'=A'Q                                                  (30)

However, the rotating frame picture is not clear for decay times shorterthan about a half cycle (Q=π/2). A possibly more appealing expressionfor π' in the case of small Q is the Fourier component, ##EQU14## where

    ω=[1-1/(2Q).sup.2 ].sup.1/2

Integral tables and a page of algebra give the same result as before:

    μ'=A'Q                                                  (32)

The corresponding cosine component is zero. The integration is validalso for Q-values right down to the critically damped value of one-half.

Since this rotation is about the axis B₁, from (32)

    M·B.sub.2 ≃ sin (A'Q)≃A'Q (33)

From (28) and (33), the component of M perpendicular to B_(e), or theprecessing component, is ##EQU15##

It can be shown that an approximate 90° phase shift in the oscillatingfield can for small A cause the terms combined in (34) to add linearlyinstead of quadratically. The result is to favor somewhat the small-Gcomponents of signal, namely, the signal from farthest out in theformation.

To compute the cutoff efficiency E from (9), now is constant for smallA, and that (9) and (34) give ##EQU16## The validity of (35) requiresthat A<<1 and also μ<<μ_(m), or from (32), AQ<<μm Thus, (35) requires

    A<<1

and

    A<<μ.sub.m /Q                                           (36)

NUMERICAL RESULTS FOR ABOVE DESCRIBED SIMPLE PARALLEL AND SERIESCIRCUITS

Through conventional equations of motion for the polarization, numericalcomputations for the modes of cutoff, given by (20), (22), and (23) forthe parallel circuit of FIG. 7, and by (25), (26), and (27) for theseries circuit of FIG. 8, have been done. Since the case with theearth's field parallel the borehole axis G=A, is only considered, thesesymbols can be used interchangeably.

The summary of results is as follows. The maximum cutoff efficiency isfor A≃μ_(m) /Q, where μ_(m) =2.33, for Q≧√2. The cutoff efficiency canbe at least E_(m) =0.725 at any A by appropriate choice of Q (orappropriate μ-value obtained by other means, i.e., allowing a tuned NMLcoil to ring with appropriate Q following voltage-limited polarizingcurrent cutoff provides a signal at least 0.725 as great as obtainedwith instantaneous cutoff. The appropriate Q to maximize signalsensitivity of the coil circuit is of the order of 2.33/G, where G isthe cutoff rate, (B_(p) /B_(e))(ωT), where B_(p) is the polarizing fieldstrength, B_(e) is the earth's field, and T is the cutoff time. For A<1an efficiency of E_(m) is obtainable, and at an A-value of 2.5 anE-value of about 0.80 can be obtained by appropriate choice of Q forsimple series or parallel circuits. The initial slope of E as a functionof A is (1/2)√1+Q² for the series circuit at any A and for the parallelcircuit to a reasonable approximation for A greater than about √2. Ifthe Q during cutoff is determined by a resistor in parallel with thetuned coil, Q-values approaching that for critical damping (Q=1/2) areto be avoided.

θ and ψ tend to oscillate at an angular rate 0.6+Q as A is increased. θtends to oscillate about π/2 with maxima and minima at multiples ofA=π/(0.6+Q). ψ oscillates with maxima and minima at odd multiples ofhalf this value.

FIG. 9 shows cutoff efficiencies for a long coil centered in theborehole for various Q values via curves 70. Note that, fortunately, agiven Q-value gives reasonable efficiency over a fairly wide range ofA'. Since A'tends to decrease as the inverse square of borehole radius,the illustrated range allows a substantial variation of borehole sizeand angle between earth's field and borehole axis without necessity ofadjusting the ringing Q.

DEPOLARIZATION TO MINIMIZE RESIDUAL POLARIZATION IN A NEXT-IN-TIMECOLLECTION CYCLE

In the prior sections, the responses of polarization M have beendescribed. In this section, the responses of polarization, includingresidual polarization to various magnetic fields, will be discussed forthe purpose of showing that for coil circuits in which higher Q valuesthan normal in conventional NML are used, components of residualpolarization parallel to the earth's field (B_(e)) at the start of thenext-in-time polarization period can be cancelled with only a slightreduction is signal. Hence, insertion of a depolarization period,between one or more of the collection cycles, is not needed.

In this regard, polarization (M), of course, can be manipulated also beresonant magnetic fields, as previously indicated.

The effects of fields near the precession, or "Larmor" frequency, can bevisualized by considering the system from a rotating reference framepreviously mentioned. If the polarization M is considered from theviewpoint of a reference frame rotating about a static magnetic field atthe precession frequency corresponding to the field, the system appearsto behave as if the field were removed. That is, the observer isrotating with the polarization; so it appears to him to be standingwill. The effect of a magnetic field rotating at or near the precessionfrequency can now be visualized. The rotating field simply looks like astatic field in the rotating reference frame. The polarization simplyprecesses in this field. But if the frequency is not exactly theprecession frequency, then a small part of the original field isuncancelled.

FIGS. 10 and 11 illustrate how effects due to components of aprior-in-time residual polarization may be cancelled.

In FIG. 10, the possibility of precessing polarization from aprior-in-time collection cycle surviving in a present-in-time collectioncycle is illustrated. In the FIG., three curves 71, 72, and 73 representportions of a present-in-time NML signal of interest as a function ofdifferent coil Q values. Curve 71 is the signal of the present-in-timepolarization which is the desired one to be maximized (with minimumcontributions to be added via signal portions associated with curves 72and 73. In this regard, curve 72 is the signal portion associated withthe first previous prior-in-time polarizing period, while curve 73 isthe signal portion from the second prior-in-time polarizing period.Assuming a set of polarizing times equal to 2000, 100, and 400milliseconds, the most series carry over is, of course, from the 2000-to 100-millisecond polarizing periods irrespective of the largerrelative carry over of the second prior-in-time period, for the reasonspreviously indicated.

In more detail, ignoring relaxation, the signal from the nth previouspolarization is proportional to (ρ cos θ)^(n). If E_(n) (A) is definedas the signal from the n^(th) previous polarization to that of thepresent polarization, neglecting relaxation, the cutoff efficiency is:##EQU17## The cos θ factor gives some cancellation in Equation (37) forodd powers of n, whereas for even n, all contributions add so long as ψdoes not vary drastically. The large ψ variations occur outside therange of plausible tool design.

But by requiring a higher than normal coil Q, there is provided a meansfor cancellation of signal components of the prior-in-time polarizationwithout significant loss in signal strength associated with thepresent-in-time cycle. That is to say, returning to FIG. 10 byspecifying a Q of 2.12 associated with dotted line B, only a 2% decreasein signal strength occurs for the present-in-time signal depicted incurve 71, but because line B intersects the zero crossing point 77 ofthe signal portion of curve 72 (that is, the signal associated withcurve 72), as much signal of curve 72 is in phase with thepresent-in-time signal 71 of interest as is produced in the oppositephase. Thus, the unwanted signal contributions of the prior-in-timesignal 72 due to components of the residual polarization that wereparallel to the earth's field at the start of the present-in-timepolarizing period, are entirely cancelled.

The shifting of the coil Q from a normal Q of 1.7 associated with dottedline A, to the higher Q of 2.12 associated with dotted line B, is basedin part (i) on the discovery that the signal strength associated withthe present-in-time polarization period is asymmetric as a function of Qvalues associated with the coil circuit. Moreover, the slope 79 oftrailing edge of the curve 71 has been found to be surprisingly shallow,at least over segment 80. (The definition of segment 80: that portion ofthe curve 71 that extends from a Q associated with maximum signalstrength to a Q value that is 2% down from that maximum.)

Note from FIG. 10 that the increase in Q, viz., from the Q valueassociated with dotted line A to that associated with dotted line B isin a range of 18 to 35% of the former Q value. An increase of about 25%is preferred.

Absolute range of increasing the Q of the coil circuit during ringingdepends on a number of factors foremost of which is the normalizedmaximum Q previously established. I.e., if the maximum Q has beenestablished at 1.7, then increasing the Q of the coil to a range of 2.0to 2.4 provides for adequate cancellation of the components of theprior-in-time polarization. A Q value of 2.1 is preferred.

FIG. 11 illustrates that Q values outside the desired range can affectthe strength of the present-in-time signal.

In the Figure, for a Q of 1.7 associated with dotted line A of FIG. 10,note that the signal portion associated with the prior-in-timepolarization, viz., curve 72 of FIG. 10, is illustrated as waveform 85.Note the large variation of amplitudes of the waveform 85 as a functionof time can make an unwanted contribution to the present-in-time signalassociated with curve 71 of FIG. 10, as previously indicated. On theother hand, for a Q of 2.12 associated with the dotted line B of FIG.10, the same signal, viz., that associated with the curve 72, can bedepicted as waveform 86. Note that its amplitude is random with time.Result: little or no contribution to the present-in-time NML signal ofinterest. Note further that too high a Q value is also not desirable. Inthe case of a Q value of 2.6, associated with line C in FIG. 10, eventhough the depicted waveform 87 has phase reversed with respect to thatof waveform 85, the former still would make an unwanted contribution tothe present-in-time signal associated with curve 71 of FIG. 10.

In order to establish the correct coil Q value, the NML tool is placedin a calibration tank, in which the coil of the polarizing and detectioncircuitry is surrounded by a section of sand or the like, containingentrained fluids, such as water. There are two basic ways to establishthe artificially higher Q' for the coil and its associated circuitelements and both occur at zero crossing point 77 of the curve 72, butwithin segment 80 of curve 71 of FIG. 10.

(1) Tune the coil to a Q value that generates an oscillating fieldduring ringing that maximizes the NML signal associated with nuclearpolarization established by the dipole moments of the entrained fluidnuclei by a prior-in-time polarizing field of predeterminedcharacteristics. Detection occurs after the polarizing field has beencutoff and ringing of the coil circuit has terminated. Then the Q of thecoil and associated elements are mathematically increased a preselectedamount as explained below, to establish an artificially high Q' valueand thereby bring about cancellation of the effects of residualpolarization; or

(2) Establish the particular duration of polarizing period of the seriesof collection cycles most likely to generate residual polarization in asubsequent collection cycle. Generate a polarizing field of the mostlikely time duration to cause a problem. After cutoff of the field,permit the coil and the associated elements of the polarizing anddetection circuitry to ring at a frequency related to the protonprecession frequency of the entrained fluid to enhance the generatedpolarization. Generate a second brief polarizing field of less strengthand duration than the initial polarizing field. Cutoff the brief fieldand allow the coil and associated elements to ring a second time.Determine the particular artificially high Q' value that minimizes theNML signal detected after both cutoff of the brief field and terminationof coil ringing, has occurred.

BRIEF DESCRIPTION OF METHOD (1), SUPRA

The purpose of method (1): to establish an artificially high Q' valuefor the coil circuit of the polarizing and detection circuitry of an NMLtool so as to reduce the effects of residual polarization in nuclearmagnetic logging (NML) operations. In that way, a series of collectioncycles normalized to a common depth interval can be carried out moreswiftly and accurately than in conventional NML operations. Theselection criterion for Q': It must be greater than that which maximizesNML precessional signal response after termination of a polarizationfield. But also it must be of a value depictable on a NML signalstrength vs. Q plot of the most pertinent collection cycle thatcoincides with the zero crossing point of a portion of a particularpresent-in-time NML signal so as to minimize the effect of the latter.The pertinent cycle has been previously determined based on which cycleis most likely to be influenced by the effects of residual polarizationleft over from the prior-in-time collection cycle. The particularresidual polarization exists because of the long time duration of theprior-in-time polarizing period and thus would be most likely toinfluence the NML signal generated in a later in time collection cycleof interest. The zero crossing point identifies phase reversal of theportion of the NML signal of interest.

Now in more detail, steps of Method (1) include the following:

(a) After the tool has been located within the test tank, a polarizingfield having a known time duration, is generated by the polarizing anddetection circuitry by driving its associated polarizing coil with anelectrical signal of known characteristics;

(b) The electrical signal is then cut off after the time duration ofstep (a) has elapsed;

(c) Then the coil and associated elements are permitted to ring at afrequency related to the proton precession frequency of entrained fluidscommon to the adjacent formation to be surveyed, to generate a decayingoscillating resonant magnetic field that propagates outwardly andreorients with enhanced results, the nuclear polarization associatedwith the generated polarizing field prior to cutoff;

(d) Next the Q of the coil and its associated elements that maximizesthe NML signal response to the enhanced reoriented polarization of step(c), is determined;

(e) Finally the Q of the coil and its associated elements ismathematically increased a selected amount to an artificially higher Q'value, based on the Q value of step (d), the artificially higher Q'value coinciding with the zero crossing point of a portion of a detectedNML that is most likely to influence the NML signal of the subsequentcollection cycle of interest. It should be noted that the increase inamount can be a simple percentage increase to achieve the higher Q' ofstep (e). In this regard, the range of increase is 18 to 35% normalizedto said Q value in step (d). An increase of about 25% normalized to theQ value for maximum NML response of step (d), is preferred. Note also inthe determination of Q' of the coil and its associated elements inaccordance with step (e) that the NML signal strength vs. Q plot isasymmetric about the Q values that produce the NML response. It also hasa trailing segment whose slope measured from a Q of maximum response inaccordance with step (d) to said artificially high Q' of step (e) issurprisingly shallow.

As previously indicated, the coil and its associated elements comprisinga portion of the polarizing and detection circuitry during ring down,are connected in circuit with each other so as to provide damping of theoscillating resonant magnetic field radiating from said coil. Suchcircuit configuration comprises a resistive element in either seriesconnection or parallel connection with a capacitor that is itselfparallel to said resistance element, as shown in FIGS. 7 and 8. Ofcourse, the coil and the associated elements themselves define the Q'value of step (e) and have particular values related to the protonresonant angular frequency (ω) of the entrained fluids in accordancewith,

Resonant frequency

    (ω)=[1-1/(2Q').sup.2 ].sup.1/2

where Q' is the artificial high quality value determined by step (e), aspreviously mentioned.

If the coil and its elements are connected in a series dampingconfiguration, as shown in FIG. 8, note that the resistive element is inseries with said capacitor but that the capacitor and the resistiveelement are themselves parallel to the coil. And the artificially higherQ' of step (e) is established by decreasing the resistance value of theresistive element in series with said capacitor from a value previouslyused to establish the Q value for maximum NML response, to a lowerresistance value. However, if the coil and its circuit elements areconnected in parallel damping configuration, as shown in FIG. 7, notethat the previously mentioned resistive element would now be in parallelwith both the capacitor and the coil, and that the artificially higherQ' of step (e) is established by increasing the resistance value of suchresistive element from that previously used to establish said Q valuefor maximum NML response.

BRIEF DESCRIPTION OF METHOD (2), SUPRA

The purpose of Method (2): to establish an artificially high Q' for thecoil circuit of the polarizing and detection circuitry of an NML tool soas to reduce the effects of residual polarization in nuclear magneticlogging (NML) operations. In that way, a series of collection cyclesnormalized to a common depth interval can be carried out more swiftlyand accurately than in conventional NML operations. The selectioncriterion of Q': It must be a value on the signal strength vs. Q plot ofthe most pertinent collection cycle that coincides with the zerocrossing point of a portion of a particular present-in-time NML signalso as to minimize the effect of the latter. The pertinent cycle has apreviously determined base on which cycle is most likely to beinfluenced by the effects of residual polarization left over from aprior-in-time collection cycle. The particular residual polarizationexists because of the long time duration of the prior-in-time polarizingperiod and thus would be most likely to influence the NML signalgenerated in a later in time collection cycle of interest. The zerocrossing point identifies phase reversal of the portion of the NMLsignal of interest where the later would have minimum effect.

Now is more detail, steps of Method (2) include the following:

(a) After the tool has been or its to be located in the test tank, thetime durations of the polarizing periods of a particular set ofcollection cycles to be normalized to a given depth interval are firstanalyzed to determine which of them is most likely to generate residualpolarization that will affect the NML signal of a subsequent collectioncycle of interest;

(b) Next with the tool and polarizing and detection circuitry residingwithin the test tank, a polarizing field having the time durationdetermined from step (a), is generated by the polarizing and detectioncircuitry by driving its associated polarizing coil with an electricalsignal of known characteristics;

(c) The electrical signal is then cut off after the time duration ofstep (a) has elapsed;

(d) The coil and the associated elements is permitted to ring at afrequency related to the proton precession frequency of entrained fluidscommon to the adjacent formation to be surveyed, to generate a decayingoscillating resonant magnetic field that propagates outwardly andreorients with enhanced results, the nuclear polarization associatedwith the generated polarizing field prior to cutoff;

(e) After the elapse of a short time period, say equal to that which isconventional for detection of precessing NML signals, a second briefpolarizing field of less strength and duration than that of step (b) isgenerated but which has a slow rising amplitude vs. time turn-oncharacteristic so as to reorient components of the polarization of step(d) that are residual after cession of said conventional time period. Inthis regard, the change in direction of the brief field during turn-onis adiabatic. That is, the instantaneous angular frequency of rotationof the field (Ω) is much less than the instantaneous precessionfrequency (ω) of the residual polarization, viz., Ω<<ω;

(f) After cutting off said brief field of step (e) and allowing the coiland the associated elements for the same Q of step (d) to ring andgenerate a second oscillating field that realigns said residualcomponents of polarization of step (e) in an enhanced orientationrelative to the earth's field, a NML signal due to precession of saidresidual components relative to the earth's field, is detected;

(g) Then steps (b)-(f) are repeated using different Q values until thedetected NML signal due to the residual components of polarization for aparticular Q' value has been minimized, whereby when said NML operationsat said Q' value normalized to said given depth interval within theformation occur, residual polarization due to the most likely polarizingperiod of step (a) will not influence present-in-time detected NMLsignals. Note that the portion of NML signal due to the prior-in-timeresidual polarization associated with said prior-in-time polarizationdoes not affect the NML signal in the collection cycle of interestbecause about as much of such portion is in phase therewith as is ofopposite phase. That is, the dynamic positions of components of theresidual polarization within the formation as detection of thepresent-in-time NML signal occurs, are equalized. Hence, the loggingoperations can be swiftly and accurately carried out without need of adepolarization period between any of the collection cycles.

Errors in the establishing of the correct Q value have also beeninvestigated.

For this purpose, viewing the system from the rotating frame, asdescribed earlier, is in order. In this instance, the rotating field dueto the ringing is 1/2 AB_(e), exp(-ω_(o) t/2Q), where A is cutoff rate,B_(e) is the earth's field, here assumed perpendicular to the polarizingfield, ω_(o) is the precession angular frequency in the earth's field,and t is cutoff time. If there is a tuning error, in addition to thisfield, a field, -B_(e) D, exists where D is the relative tuning error.That is, if the coil circuit is tuned 2% low, then 2% of the earth'sfield is not cancelled by our being on the rotating reference frame. Thecutoff-rate factor for the decaying field of the ringing in thepolarizing coil is then ##EQU18## where the parameters can be shown inanalogy with FIG. 3 with φ_(e) =90°. If Q and D are small, α can belarge over much of the range of φ. However, with a 2%-tuning error and aQ of ten, the maximum value of α would be slightly less than one. Byanalogy with FIG. 3, note that, if the tuning is low, the angle betweenthe polarization and the earth's field would be reduced by the tuningerror. But, if the tuning is high, the angle is increased.

FIG. 12 shows θ as functions of A for Q=8 and for tuning 2% low,correct, and 2% high.

MODIFICATION

While the effects of residual polarization has been reduced by usingcoil circuits having artificially high Q' values, instances can occurthat sometimes mitigate against optimum performance. For instances,after a higher Q value for a given coil circuit has been establishedusing a test tank (Criterion: choosing a Q value which minimizesresidual polarization strength as previously described), there can stillarise unexpected problems having to do with unexpected changes as theNML sonde is operated within an downhole environment.

For example, there can be a lack of precision in the frequency tuningprocedure accompanying the NML process. Also, variations in the earth'sfield due to stratography can occur from depth to depth along thewellbore and lower or raise the precession frequency a slight amount.Result: slight tuning errors in the coil circuit can be introducedduring ringing that can influence the next-in-time NML signal due to theeffects of the error magnetic field in the rotating reference frame.

Key to the present discovery: at Q values normally used in prior NMLcoil circuits that maximize NML detection response, I have found thatslight tuning errors, say due to environment, do not have a greateffect. But such effects can be of more importance in coil circuitsemploying artificially high Q' values.

In order to reduce the effect due to tuning errors, viz., a magneticerror field -B_(e) D, I have discovered for large values of A, say thosewhich have more influence at the more remote positions in the formationunder survey, that most of the decay, viz., during ringing of the coilcircuit, takes place before there is much in increase in the φ where φis the angle betwen the polarizing field (Bp) and the resultant field(B). Hence, after ringing for a few time constants at the artificiallyhigh Q has occurred, the present invention teaches that a low Q valuefor the coil circuit can then be dynamically reestablished in thatcircuit. Result: Because of my discovery that cutoff rate is inverselyproportional to the Q of the coil circuit (and hence at lower values ofthe latter, viz., high cutoff rates, the polarization is not influencedby the error field), I have found that reestablishing ringing at a lowerQ value after most of the decay at the artificially higher Q' value hasoccurred, viz., in a time frame in which angle φ has not changed muchequivalent to a few time constants of decay, maintains the effect ofring down at the higher Q' value but without the consequences of slowcutoff of the decaying field. Result: the effect is as if ringing is atthe higher Q' value but at a high cutoff rate associated with a lower Qvalue for the coil circuit. And the accentuated effect due of the tuningerror, viz., due to the tuning field, -B_(e) D at the higher Q value, isavoided. That is to say, ringing at the higher Q value for the firstcouple of time constants after cutoff still provides for thecancellation of the residual polarization about the zero crossing pointof the signal in the manner of FIG. 10 but because the lower Q value ofthe coil circuit becomes effective before the consequence of slowcutoff, the field due to the relative tuning error is a function of thelower Q value only and does not greatly influence the next-in-time NMLsignal.

FIG. 13 illustrates the modification of the polarizing and detectioncircuit 13 of FIG. 2 in accordance with the present invention.

As shown, the coil 35 is still connected uphole to the uphole system 19via: (i) on one side by conductor segment 36a, switch 37 and upholeconductor 18c and (ii) on the other side, by conductor 18b. In serieswith conductor segment 36a are conductor segments 36b-36g. The coil 35is connected to ground at 39 by means of additional conductor segments40a-40e. Between the segments 36d and 36e is Q-lowering resistor 200.The resistor 200 is a circuit addition to the elements previouslydescribed (viz., Zener diodes 42, resistance element 43 and capacitor 44all parallel to coil 35). Switch 201 disconnectably connects theQ-lowering resistor 200 to ground 39 on command via an activation signalfrom the uphole system 19 that closes the contacts of the latter. Notethat the command signal to switch 201 from the system 19 is transmittedby conductor 18h.

In operations, during polarization the switches 37, 45 and 46 operate aspreviously described. That is, the contacts of switch 37 are closedwhile those of switches 45 and 46 are open. More importantly thecontacts of switch 201 are also open. However, after cutoff occurs,i.e., the contacts of switch 37 are opened whole those of switch 45 areclosed, the contacts of switch 201 also remain open. Ringing of coilcircuit then occurs at the artificial higher Q value until about twotime constants have passed as measured by the uphole system 19. Then acommand signal via conductor 18h causes the contacts of switch 201 toclose so as to lower to the Q of the coil circuit.

Because the cutoff rate is surprisingly inversely proportional to the Q(and hence at lower Q values the cutoff rate is high so that thepolarization does not follow the magnetic field created by the tuningerror during ringing) the angle φ between the polarizing field and theresultant field does not change much. So the advantage of ringing athigh Q can be maintained but by switching the coil circuit to a low Qvalue, the effect of mistuning at such a high Q is reduced. Thus, theconsequences of slow cutoff are avoided, especially for large cutoffrates (A's) having influence at more remote locations within theformation. (In the Definitions Section, the dependence of signalsensitivity on A is discussed in detail).

FIG. 14 shows the effect of lowering the Q of the coil circuit afterseveral time constants in more detail.

As shown, for large A's, most of the decay takes place before there ismuch decrease in φ. Thus, the effect of mistuning can be decreased byswitching Q from a high value to a low value. The result of the effectof the ringing at a high Q is maintained, but without the consequencesof slow cutoff. I.e., FIG. 14 shows the computation for ringinginitially at a Q of 8, but with Q reduced to 1 after two time constantsof decay. The effects of mistuning are reduced for any A and nearlyeliminated for large A's in any range beyond 1.7.

However, note that the results of FIG. 14 assume that ringing is roughlyproportional to the terms in the equation, AQ (1-e^(-m)) where m isnumber of time constants of ringing that pass before the low Q value forreducing the effects of tuning errors is inserted in the coil circuit.That is, nuclear polarization generated in the adjacent subsurface isinfluenced--with enhanced results--in an amount associated with thedamped product AQ that is directly associated with the angle of rotation(μ), turned by the rotating reference frame in AQ radians as a functionof the term (1-e^(-m)).

In investigating the effects of changes in the time span of ringing, ithas found that if m=2, there is a substantial reduction in signal errorsdue to reducing the effect of small tuning faults. Moreover, aspreviously indicated in conjunction with FIG. 14, the mitigation of sucheffects is substantial for cutoff rates (A's) above 1.7. But if thecutoff rates are lower than say A=1.7, then my investigation also showsthat m in the above-identified equation should probably be made equal to1 coupled with an upward change in the value of the low Q of the coilcircuit, viz., the latter should be boosted to make up for the reductionin the factor (1-e^(-m)). Moreover, for very low A's, m should probablybe made to assume a value of less than one, say about 1/2.

In other words, under circumstances where the NML procedure uses lowerA's, it is desirable that the method of the present invention bemodified so that ringing would be more short-lived before the low Qvalue is established in the coil circuit in order to reduce the effectsof tuning errors. That is, the present method should be modified so thatthe cutoff rate is prevented from decaying as far as it previously did.The result is to provide the most desired angle of rotation of therotating frame of reference vis-a-vis other factors of concern in themanner previously described.

SYMBOLS AND DEFINITIONS SECTION

For convenience, the unit of time is here the reciprocal precessionangular frequency in the earth's field (the full earth's field, notmerely the component perpendicular to the polarizing field). The unit ofangular frequency is the precession angular frequency in the earth'sfield; the unit of field strength is the strength of the earth's field.

ω instantaneous local precession angular frequency about the resultantfield (vector sum of the earth's field and whatever field is produce ata given time and place by the polarizing coil or whatever coil is beingconsidered).

Ω instantaneous local angular frequency of turning of the resultantfield (rate-of-change of direction irrespective of amplitude).

α=Ω/ω.

R the ratio of the component of the resultant field parallel to thepolarizing field to the component of the earth's field perpendicular tothe polarizing field.

φ=cot⁻¹ R; φ_(e) is φ when the polarizing field has been reduced tozero, namely, the angle between the earth's field and the polarizingfield.

θ the instantaneous local angle between the resultant field and thepolarization.

Note: A prime (') will frequently be used to indicate some quantity atan arbitrary distance into the formation, whereas the symbol without theprime will indicate the quantity at the edge of the borehole.

A the instantaneous value of dφ/dt (in units mentioned above) at thetime during cutoff at constant rate (or extrapolation thereof) when theresultant field is perpendicular to the polarizing field. A=-(dR/dt) sinφ_(e).

G=-(dR/dt) sin φ_(e). Note that dt is in units of reciprocal precessionangular frequency in the earth's field and represents the quantityγB_(e) dt in more general units. Thus, G=dB_(p) /dt in our units. G isalso the polarizing field (in units of the earth's field) divided by thecutoff time (in units of reciprocal precession angular frequency in theearth's field) for constant cutoff rate. G=A sin² φ_(e). Note that A andG are the same when the polarizing field is perpendicular to the earth'sfield (either locally or, in the case in which the earth's field isparallel to the borehole axis, substantially everywhere).

B₁ the unit vector in the direction B_(e) ×B_(p).

B₂ =B₁ ×B_(e).

ρ distance from the borehole axis.

a borehole radius.

μ angle of rotation in the rotating frame of reference about some axisperpendicular to the earth's field.

E polarizing field cutoff efficiency neglecting relaxation effects, theratio of the signal following some particular mode of polarizing fieldcutoff to the signal for instantaneous cutoff. Note that it is notimpossible for E to be greater than 1.0.

SIGNAL SENSITIVITY DEPENDENCE ON A

The simpliest NML field computation is for the centered "ideal coil",which produces approximately a two-dimensional dipole field. This coilmust be long compared to the borehole diameter. The field isperpendicular to the borehole axis, which we will assume to be thedirection of the earth's field. The field B is inversely proportional tothe square of distance from the axis. Apart from the angles θ and ψassociated with polarizing field cutoff, the signal contribution from asmall volume element is proportional to B². One factor is for thestrength of polarization produced by the field, and the other is for thecoupling of the field of the precessing polarization back into the NMLcoil. Since our computations are for ratios of sensitivities, we willnot be concerned with constant multipliers. The sensitivity per unitaxial length is proportional to

    ds˜B.sup.2 ρdρ˜dρ/ρ.sup.3      (A- 1)

The factor of ρdρ is proportional to the volume element per unit length.The cutoff rate A is also proportional to B. If A_(oo) is the cutoffrate at the edge of a borehole of radius a for our ideal coil system,

    A=A.sub.oo a.sup.2 /ρ.sup.2                            (A- 2)

Differentiating (A-2) and comparing with (A-1) gives

    dS˜dA                                                (A-3)

This corresponds to f(A)=1 in Equations 4, 5 and 9. If the coil is notlong compared to the borehole diameter, the field drops off faster than1/ρ² for large ρ, eventually dropping off as 1/ρ³ as for athree-dimensional dipole. We will compute the field for a line dipoleextending from z=-b to z=+b. We limit the computation to the x-directionin the z=0 plane. The field is ##EQU19## We now define

    u=A.sub.oo a.sup.2 /x.sup.2

    G=A.sub.oo a.sup.2 /b.sup.2,                               (A-5)

where A_(oo) is defined in Equation A-2 for the long coil. We no longerhave u=A, however. We now have

    B˜A=(u+2G)/(1+G/u).sup.3/2                           (A- 6)

We still have dS˜B² du/u², giving

    f(A)dA=dS=u(u+2G).sup.2 du/(u+G).sup.3                     (A- 7)

As an example, if the borehole diameter is half the coil length andA_(oo) =1.6 is the cutoff rate at the edge of the borehole for a longcoil system with the same winding cross section and current as the shortcoil, Equation A-2 gives G=0.4, and Equation A-6 gives A_(o) =1.72. Itmay at first be surprising that the near field of a short line dipole isgreater than that of a long one. However, the field from the moredistant part opposes that from the part near the z=0 plane.

All specific embodiments of the invention have been described in detail,and it should be understood that the invention is not limited thereto asmany variations will be readily apparent to those skilled in the art.FIG. 9 also shows that cutoff efficiency E is a dependent variable, andthat the dependent variable A varies as function of the coil Q. Hence,instead of increasing the coil Q until it is approximately at the zerocrossing of the prior-in-time polarization to cancel the latter, it isalso possible to duplicate that result via manipulation of thedimensionless parameter A. This could be achieved by increasing thebreakdown voltage of the pair of Zener diodes 64 and 69 of FIGS. 7 and8, respectively, to approximately double that associated with the higherQ coil circuit so that the amplitude of oscillations during ringingwould double. Result: the prior-in-time polarization are centered at itszero crossing point with the cancellation as previously describedoccurring. It should be noted that while the present invention dictatesthat the contacts of switch 45 of FIG. 2 connecting resistance element43 in circuit with the coil 35 be open during polarization to maximizingthe driving voltage to the polarizing coil 35, the amount may in somecases be so small as to be unimportant. Moreover, if the resistiveelement 43 is in series with capacitor 44, no loss in power can occurduring polarization irrespective of the condition of the switch 45.

What is claimed is:
 1. Method for reducing the effects of residualpolarization as well as mitigating tuning errirs during ring down asnuclear magnetic logging (NML) operations occur so that a series ofcollection cycles normalized to a common depth interval can be carriedout more swiftly and accurately than in conventional NML operations,wherein the common depth interval lies within an earth formationpenetrated by a wellbore adjacent to NML polarizing and detectioncircuitry positioned within the wellbore under control of NMLcomputer-linked controller and recording system at the earth's surface,and wherein entrained fluids with the common depth interval arerepetitively polarized with a polarizing field (B_(p)) at an angle tothe earth's field (B_(e)), and after the polarizing field has beencutoff, NML signals from precessing protons of fluid nuclei within theformation are detected, comprising:(i) establishing an artificially highQ' value for the polarizing and detection circuitry that is greater thanthe normal Q value which maximizes NML precessional signal responsewherein said artificial Q' value coincides with the zero crossing pointof a portion of a present-in-time NML signal of interest, said portionbeing associated with a prior-in-time polarizing period that because ofits characteristics is most likely to influence the NML signal generatedin the present-in-time collection cycle of interest, said zero crossingpoint identifying phase reversal of said portion of said NML signal;(ii) positioning the polarizing and detection circuitry within thewellbore adjacent to a common depth interval; (iii) repetitivelypolarizing proton of fluids within said common interval by a polarizingfield (B_(p)) to define a series of collection cycles that includes saidpresent-in-time and prior-in-time collection cycles, during eachcollection cycle said polarizing field realigning dipole moments of thefluid nuclei and forming a nuclear polarization at an angle to theearth's field; (iv) terminating each polarizing period after a knowntime duration by cutting off the polarizing field and permitting a coilcircuit of said polarizing and detection circuitry to ring at saidhigher artificially higher Q' value; (v) after ringing said coil circuitat said higher artificially higher Q' value for a limited time,establishing a lower Q value for said coil circuit and permittingringing to continue at said lower Q value wherein the effect due totuning errors is reduced but wherein the reduction of residualpolarization due to phase cancellation is maintained. (vi) detecting theprecessing nuclear polarizations of the series of collection cycles as aseries of NML signals, said series of collection cycles includes saidpresent-in-time and said prior-in-time collection cycles whereby thepresent-in-time NML signal is not influenced by said signal portiongenerated by the residual polarization because of phase cancellationtherewith whereby said logging operations can be swiftly and accuratelycarried out without need of a depolarization period between saidnext-in-time and prior-in-time collection cycles.
 2. Method of claim 1in which the ringing of the coil circuit at the higher Q' value inaccordance with step (iv) is for about 2 time constants of the decayingfield.
 3. Method of claim 1 in which the ringing of the coil circuit atthe higher Q' value in accordance with step (iv) is for about one timeconstant of the decaying field.
 4. Method of claim 1 in which theringing of the coil circuit at the higher Q' value in accordance withstep (iv) is for about 1/2 time constant of the decaying field. 5.Method of claim 1 in which said NML signal portion that is generated bythe prior-in-time residual polarization is defined by

    ρ(cos θ).sup.n sin θe.sup.iψ f(A')dA'

where n is equal to 1, ρ is the distance from the borehole axis; θ isthe instantaneous local angle between the resultant field (B) and thepolarization M, ψ is the angle about the resultant field (B) withrespect to the plane of the polarization field (B_(p)) and the earth'sfield (B_(e)), A' is the instantaneous value of dφ/dt at the time duringcutoff at a constant rate (or extrapolation thereof) when the resultantfield (B) is perpendicular to the polarizing field, Φ=cot⁻¹ R where R isthe ratio of the component of the resultant field (B) parallel to thepolarizing field (B_(p)) to the component of the earth's field (B_(e))perpendicular to the polarizing field (B_(p));and wherein cancellationof said NML signal portion generated by the prior-in-time residualpolarization, occurs because as much of said signal portion is in phasewith said present-in-time signal as in opposite phase therewith, wherebysaid logging operations can be swiftly and accurately carried outwithout need of a depolarization period between said present-in-time andprior-in-time collection cycles.
 6. Method of claim 1 in which step (v)is further characterized by the effects of tuning errors beingsubstantially eliminated for substantially all cutoff rates (A's) of thedecaying field.
 7. Method of claim 1 in which said higher Q' and lower Qvalues for said coil circuit during ring down are established by thesubsteps of:(a) analyzing the time durations of the polarizing periodsof the series of collection cycles to be normalized to a given depthinterval to determine which of the prior-in-time polarizing periods ismost likely to generate residual polarization that will affect the NMLsignal of a subsequent collection cycle; (b) generating a polarizingfield having the time duration determined from step (a), by driving apolarizing coil and associated elements of said polarizing and detectioncircuitry with an electrical signal of known characteristics; (c)cutting off the signal after the time duration of step (a) has elapsed;(d) permitting the coil and the associated elements to ring at theproton precession frequency of entrained fluids common to the adjacentformation to be surveyed, to generate a decaying oscillating resonantmagnetic field that propagates outwardly and reorients the nuclearpolarization associated with said polarizing field of step (b) withenhanced results, said coil and associated elements defining said normalQ value during ringing; (e) after the elapse of a short time period,generating a second brief polarizing field of less duration than that ofstep (b) but having a slow rising amplitude vs. time turn-oncharacteristic so as to reorient components of the polarization of step(d) that are residual after cession of said conventional time period;(f) cutting off said brief field of step (e) and allowing the coil andthe associated elements for the same Q of step (d) to ring and generatea second oscillating field that realigns said residual components ofpolarization of step (e) in an enhanced orientation relative to theearth's field; (g) detecting a NML signal due to precession of saidresidual components relative to the earth's field; (h) repeating steps(b)-(g) using different Q values until the detected NML signal due tothe residual components of polarization has been minimized, said final Qvalue being said artificially higher Q' value.
 8. Method of claim 1 inwhich said higher Q' and lower Q values for said coil circuit duringring down are established by the substeps of:(a) generating a polarizingfield having a known time duration by driving a polarizing coil of saidpolarizing and detection circuitry with an electrical signal of knowncharacteristics; (b) cutting off the signal after the time duration ofstep (a) has elapsed; (c) permitting the coil and associated elements toring at the proton precession frequency of entrained fluids common tothe adjacent formation to be surveyed, to generate a decayingoscillating resonant magnetic field that propagates outwardly andreorients with enhanced results, the nuclear polarization associatedwith said polarizing field prior to cutoff; (d) determining the Q of thecoil and its associated elements that maximizes the NML signal responseto the enhanced reoriented polarization of step (d), said determined Qvalue being said normal Q value; (e) mathematically determining theartificially higher Q' value for the coil and its associated elementsbased on said normal Q value of step (d), said artificially higher valueQ' coinciding with the zero crossing point of a portion of the detectedNML signal generated by residual polarization of a prior-in-timecollection cycle, said prior-in-time cycle being the most likely toinfluence the NML signal of the subsequent collection cycle of interest.9. Method of claim 8 in which the artificially higher Q' of step (e) isdefined by an increase of said normal Q value for step (d) in a range of18 to 35% thereof.
 10. Method of claim 8 in which the artificiallyhigher Q' value of step (d) is defined by an increase of about 25% ofsaid normal Q value of step (d).
 11. Method of claim 8 in which thedetermination of Q' of the coil and its associated elements inaccordance with step (e) defines an asymmetric amplitude vs. time NMLsignal response, as a function of different Q values, said asymmetricresponse having a trailing segment whose slope measured from a Q ofmaximum response in accordance with step (d) to said artificially highQ' of step (e) is surprisingly shallow.
 12. Method of claim 11 in whichdecrease in said NML signal response over said segment is about 2%. 13.Method of claim 8 in which said coil and its elements are connected incircuit with each other so as to provide damping of the oscillatingresonant magnetic field radiating from said coil after cutoff of saidpolarizing field and comprising a resistive element in one of seriesconnection and parallel connection with a capacitor parallel to saidresistance element, said coil and its elements having values definingsaid Q' value of step (e) and being related to the proton angularfrequency (ω) of the entrained fluids of the adjacent earth formation inaccordance with,

    Angular frequency (ω)=[1-1/[2Q').sup.2 ].sup.1/2

where Q' is the artificial high Q value determined by step (e). 14.Method of reducing the effects of both residual polarization and tuningerrors during nuclear magnetic logging (NML) operations associated witha common depth interval in an earth formation that uses repetitiveenhancement of the prior reoriented dipole moments of protons of fluidsin the formation adjacent to a wellbore penetrating the formationwherein during each collection cycle, a portion of the collapsingpolarizing field (B_(p)) after cutoff, is used to reorient said momentsrelative to the earth's field (B_(e)) using a coil circuit of apolarizing and detection circuit positioned within the wellbore undercontrol of computer-linked controller and recording system at theearth's surface, said coil circuit being caused to ring by saidcollapsing field and generate an oscillating decaying magnetic field tofavorably positioned said moments at an angle to the earth's field(B_(e)) so that after precession of the moments about the earth's fieldand generation of detectable NML signals have occurred, operations canbe carried out more swiftly and accurately than in conventional NMLoperations, comprising:(i) establishing an artificially high value (Q'),a normal Q value that maximizes NML precessional signal response afterpolarization, ring down, precession of moments of the fluid nuclei aboutthe earth's field (B_(e)) and detection of said moments, and a lower Qvalue for said coil circuit during generation of said oscillatingmagnetic field due to the collapsing polarizing field (B_(p)), said Q'value being greater than said normal Q value which maximizes saidartificial Q' value also coinciding with a zero crossing of a portion ofa present-in-time NML signal generated by residual polarization of aprior-in-time collection cycle, said signal portion being associatedwith a prior-in-time polarizing period that because of its time durationis most likely to influence NML signals generated in the present-in-timesubsequent collection cycle of interest; (ii) positioning the polarizingand detection circuit that includes said coil circuit within thewellbore adjacent to a common depth interval of interest; (iii)repetitively operating said polarizing and detection circuit includingsaid coil circuit so as to polarize protons of entrained fluids, ringdown of said coil circuit at said artificially high Q' value for a shorttime period to produce a decaying magnetic field, followed by continuedring down at said lower Q value so as to reduce tuning errors, and thendetect NML signals associated with a series of collection cycles havingdifferent polarizing periods but wherein the effects of residualpolarization have been minimized due to phase cancellation so thatlogging operations can be swiftly and accurately carried out withoutneed of a depolarization period between any of the collection cycles.15. Method of claim 14 in which the ringing of the coil circuit at thehigher Q' value in accordance with step (iii) is between about 1/2 to 2time constants of the decaying field.
 16. Method of claim 14 in whichstep (iii) includes the substeps of:(a) repetitively polarizing protonsof fluids with said common interval by a polarizing field (B_(p)) todefine a series of collection cycles that includes said prior-in-timeand present-in-time collection cycles wherein during each cycle, saiddipole moments of the nuclei are realigned relative to the earth'sfield, said realigned dipole moments generating in turn nuclearpolarization at an angle to the earth's field; (b) terminating eachpolarizing period after a known time duration by cutting off thepolarizing field, whereby a portion of the collapsing polarizing fieldcauses said polarizing coil and associated elements including the coilcircuit having a quality value of Q' to ring and generate the decayingoscillating magnetic field that affects and enhances said polarizationof step (a), and then establishing said lower Q value for the coilcircuit and continuing ringing but wherein for all cutoff rates (A's) ofsaid decaying field, the effect of tuning errors is reduced but whereinNML signals due to prior-in-time residual polarization associated withsaid prior-in-time polarization of step (i) does not affect NML signalsof the present-in-time collection cycle of interest, saidpresent-in-time NML signals not being influenced because about as muchof said NML signal portion generated by said prior-in-time residualpolarization is in phase therewith as is of opposite phase; (c)detecting the precessing polarizations associated with the series ofcollection cycles as a series of NML signals whereby said loggingoperations can be swiftly and accurately carried out without need of adepolarization period between said present-in-time and prior-in-timecollection cycles.